Violent Head-Butting Discovered in Giant Reef Fish

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Scientists diving in the North Pacific were jarred when they
heard mysterious sounds like spear guns being fired. The jolting
noises turned out to be never-before-seen (or heard) head-butting
bouts between some of the largest coral-reef fish, called
parrotfish.

As they studied groups of the giant bumphead parrotfish
(Bolbometopon muricatum), which can reach 4 feet (1.3
meters) long and 100 pounds (46 kg), the researchers were able to
document the violent behavior with video and images.

"To us, it sounded like someone was firing a spear gun off close
to our ears. Before we saw the behavior, we actually joked that
the bumpbheads were head-butting like
bighorn sheep," study researcher Roldan Muñoz, of the
National Marine Fisheries Service, Beaufort Laboratory, in North
Carolina, told LiveScience. "We were blown away when we actually
witnessed the behavior."

During the summer of 2011, during about 100 hours of snorkel and
scuba dives, the researchers witnessed several instances of the
head-butting rituals off Wake Atoll. The male parrotfish swam
head-on toward each other until they delivered their blow,
head-bump to head-bump, a collision that is painful even to
watch. Immediately after the head-on crash, the opposing
parrotfish swam rapidly in a semicircle, trying to bite the back
and flank of each other. [ Video
of bizarre head-butting parrotfish ]

"Following circling, fish swam apart in opposite directions and
then turned again face to face to initiate additional
collisions," the researchers write this week in the open-access
journal PLoS ONE.

Observations of the rowdy rituals revealed that most occurred on
the days and in the locations of spawning.

As such, the researchers suspect
sexual selection can explain the emergence of the so-called
ossified ridge, or bump, atop the male parrotfish's head, along
with the butting behavior. It's more likely these head-butting
genes will stay in the population than non-head-butters' genes
will, the researchers said.

"We believe that bumphead males
head-butt to establish dominance and access to preferred
territory that is then used to advertise to females that the
males are available for mating," Muñoz said. In fact, in videos
of the violent display, the male that wins the butting bout
hovers over a particular spot, maintaining his place in the water
column to await the females.

"No one has ever heard of any species of marine fish head-butting
until now," Muñoz said.

But how could such boisterous behavior go unnoticed for so long?

Researchers have some thoughts on that. Since the behavior is the
result of competition between males for access to the gals, it is
likely to occur only in high-density populations of the fish.
Many of the populations have dwindled and so would be unlikely to
support the behavior.

The perfect spot for head-butting? The
Great Barrier Reef, which holds the healthiest bumphead
populations along its outer reefs.

"But one would have to be at the outer reef early in the morning,
specifically trying to observe reproductive behavior, in order to
have a chance of observing head-butting — and this is no small
feat given the distance from shore of the outer reefs,"
Muñozsaid.

"At Wake Atoll, the outer reef is very close to shore, so we were
easily able to put ourselves in the right place at the right
time. But we were absolutely surprised."

The researchers plan to return to Wake Atoll to learn more about
the bumphead population's mating behaviors.